Conventionally, various suggestions are made for a cube-corner retroreflective article provided with a number of quadrilateral element pairs each formed by a pair of quadrilateral elements, in which each of the quadrilateral elements is a cube-corner element having reflective lateral surfaces including two trapezoidal reflective lateral surfaces and one pentagonal or isosceles-triangular reflective lateral surface, having a projection shape of a quadrangular when projected onto a common plane, and sharing one side (DE or AB) of the pentagonal or isosceles-triangular reflective lateral surface with the other one in the pair.
Japanese Patent Application Laid-Open No. 51-082592 (Patent Document 1), Japanese Patent Application Publication No. 52011200 (Patent Document 2), and Japanese Patent Application Publication No. 58-005401 (Patent Document 3) all of which are for inventions by Tanaka, for example, disclose that a number of quadrilateral elements, each of which is formed at an end of a pin having a quadrilateral section, are collected to form a cube-corner retroreflective article in such a manner that optical axes of the quadrilateral elements tilt so as to improve an entrance angle characteristic. As a specific amount of the tilt angle of the optical axes, 12 DEG is disclosed; however, it is not disclosed how much tilt angle is preferable. Also, it is not disclosed a quadrilateral element of which shape has an entrance angle characteristic, an observation angle characteristic, and a rotation angle characteristic that are preferable.
In addition, U.S. Pat. No. 3,833,285 (Patent Document 4), U.S. Pat. No. 3,873,184 (Patent Document 5), U.S. Pat. No. 3,923,378 (Patent Document 6), U.S. Pat. No. 3,926,402 (Patent Document 7), and U.S. Pat. No. RE29,396 (Patent Document 8), all to Heenan, disclose a method of improving an observation angle characteristic, in which deviations are provided for apex angles and disclose a combination of elements respectively having optical axes in different directions.
Further, U.S. Pat. No. 6,015,214 (Patent Document 9) and U.S. Pat. No. 6,767,102 (Patent Document 10), both to Heenan, disclose that a number of quadrilateral elements, each of which is formed at an end of a thin plate, are stacked to form a cube-corner retroreflective article in such a manner that optical axes of the quadrilateral elements tilt so as to improve an entrance angle characteristic.
FIG. 28 of Patent Document 9 shows modified examples of a shape of the quadrilateral element. A quadrilateral element having an apex of off-center (Apex Decentration) is illustrated as a first example, a quadrilateral element having a shape with a larger width (boundary proportion) is illustrated as a second example, and a quadrilateral element having an optical axis that tilts (Axis Cant) is illustrated as a third example. However, it is not disclosed a quadrilateral element of which shape has an entrance angle characteristic, an observation angle characteristic, and a rotation angle characteristic that are preferable.
In addition, U.S. Pat. No. 5,981,032 (Patent Document 11) and U.S. Pat. No. 6,302,992 (Patent Document 12) to Smith et al U.S. Pat. No. 6,889,371 (Patent Document 13) and U.S. Pat. No. 7,152,983 (Patent Document 14) to Smith, U.S. Pat. No. 6,253,442 (Patent Document 15) to Benson et al., and U.S. Pat. No. 6,318,987 (Patent Document 16) to Luttrel et al. disclose a method of forming a mold suitable for manufacturing a retroreflective article formed with quadrilateral elements by forming a cube-corner element at an end face of a thin plate (lamina) and combining a number of the thin plates.
However, it is not disclosed, in any of the patent documents, quadrilateral elements of which shape to use so as to provide an excellent retroreflective characteristic, in particular, an entrance angle characteristic and a rotation angle characteristic that are excellent.
Also, a tilt direction of an optical axis of a quadrilateral element disclosed in any of the patent documents is limited to a direction perpendicular to one side (DE or AB) of a pentagonal or isosceles-triangular reflective lateral surface of the quadrilateral element.    Patent Document 1: Japanese Patent Application Laid-Open No. 51-082592    Patent Document 2: Japanese Patent Application Publication No. 52-011200    Patent Document 3: Japanese Patent Application Publication No. 58-005401    Patent Document 4: U.S. Pat. No. 3,833,285    Patent Document 5: U.S. Pat. No. 3,873,184    Patent Document 6: U.S. Pat. No. 3,923,378    Patent Document 7: U.S. Pat. No. 3,926,402    Patent Document 8: U.S. Pat. No. RE29,396    Patent Document 9: U.S. Pat. No. 6,015,214    Patent Document 10: U.S. Pat. No. 6,767,102    Patent Document 11: U.S. Pat. No. 5,981,032    Patent Document 12: U.S. Pat. No. 6,302,992    Patent Document 13: U.S. Pat. No. 6,884,371    Patent Document 14: U.S. Pat. No. 7,152,983    Patent Document 15: U.S. Pat. No. 6,253,442    Patent Document 16: U.S. Pat. No. 6,318,987